Bietti crystalline dystrophy in Türkiye: A genetic crossroads between Asia and Europe

To analyse the genotype and phenotype characteristics of Turkish patients clinically diagnosed with Bietti Crystalline Dystrophy (BCD). The clinical findings and the genetic testing results of patients diagnosed with BCD were analyzed retrospectively. Thirty-two patients from 26 families were included in the study. Parental consanguinity was reported in 14 patients (43. 7%). All patients carried biallelic pathogenic or likely pathogenic variants in CYP4V2. In total, 12 different CYP4V2 variants were identified, three of which were novel: c. 802-1G > A, c. 802–2 A > C, and c. 464 A > C (p. His155Pro). The most common variants were c. 1198 C > T (p. Arg400Cys), with an allelic frequency of 23. 4% (15/64), and c. 802–8₈10del17insGC, with an allelic frequency of 18. 8% (12/64). Clinical features, including age of onset, visual acuity, disease stage, and optical coherence tomography (OCT) findings, did not differ significantly between genotype groups (p > 0. 05). Interocular asymmetry in visual acuity and OCT parameters, as well as variability of clinical findings between siblings and among unrelated patients carrying the same CYP4V2 variants, were observed in the cohort. This study, one of the largest European cohorts and the first comprehensive series from Türkiye, broadens the mutational spectrum of CYP4V2 with three novel variants and highlights the marked phenotypic variability of BCD, even within families. The unexpectedly high frequency of c. 802–8₈10del17insGC in the Turkish population, previously reported mainly in East Asia, provides new insights into the geographic distribution of this variant.